This work on the development of the cerebral cortex relies heavily on neuroanatomical techniques and focuses on the role of eliminatory events which occur during normal brain development. Much of our effort has concentrated on the transient visual cortical pyramidal tract which we previously identified. This normal transient projection can e at least partially maintained if the visual cortex receives somatosensory input through the lateral geniculate nucleus as a result of an induced abberant projection of the medial lemniscus, implicating the thalamus as important in determining which projection developing cortical neurons will maintain. Collateral elimination also occurs during the development of the fornix, and of the projection of the locus coeruleus. Our findings in experiments involving heterotopic cortical transplants implicate position within the tangential plane of the cortex as a critical factor in determining which of the initially extended projections, a cortical neuron will maintain.